Transmission Media

A transmission medium can be broadly defined as anything that can carry information from a source to a destination. For example, the transmission medium for two people having a dinner conversation is the air. For a written message, the transmission medium might be a mail carrier, a truck, or an airplane.

Guided Media (or Wired Media)

In this type of media, the signal energy is contained and guided within a solid media. The guided media is used for point-to-point communication. Example of guided media are Twisted pair cable, Co-axial cable and Fiber optic cable.

Unguided Media (or Wireless Media)

In the unguided media, the signal energy propagates in the form of unguided electromagnetic waves. The examples of unguided media are radio and infrared light.

Twisted-pair cables

Twisted-pair cables are usually made of copper and a pair of wires are twisted together to reduce interference by adjacent wires as shown in fig. A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together. One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference.

This means, if two wires are parallel to each other, the noise or cross talk may effect one wire and the difference between two levels will vary.

When these wires are twisted, both wire have similar effect of noise. This way, the receiver receives correct signal.

The number of twists on the cable define the quality of signals carried by them. So, more twists means better quality signals.

Twisted pair wires are generally used in local telephone communication and for digital data transmission over short distance upto 1 km.

Twisted pairs are normally used to connect terminals to the main computer up to short distances.

Data transmission speeds of up to 9600 bits per second (bps) are possible for a distance up to 100 meters.

For longer distance, speed of data transmission is 1200 bps.

Advantages

• It is easiest to install. manpower to repair and service are easily available.

•  In telephone system, signal can travel several kilometers without amplification.

• Can be used for both analog and digital data transmission.

• Least costly for short distances.

• It is flexible and easy to connect.

• It has low weight.

 Disadvantages   

1. Higher error rates when the line length is more than 100 meters because it is easily affected by more signals.

2. Being thin in size, likely to break easily.

3. It has low bandwidth.

4. It only supports data transfer rate up to 10 MBPS (Megabytes per second).

Types of Twisted Pair Cables

(a) Unshielded Twisted Pair (UTP)

(b) Shielded Twisted Pair (STP)

UTP : A twisted pair consists of two insulated conductor twisted together in the spiral form as shown in Fig. It can be shielded with plastic cover. The UTP cables are very cheap and easy to install. But they are badly affected by the noise interference.

STP :STP cable has a metal foil to cover each pair of insulating conductors. This is known as the metal shield. It reduces the interference of the noise but makes the cable bulky and expensive. So practically UTP is more used than STP.

Co-axial Cable

Co-axial cables consist of a central copper wire surrounded by a PVC insulation over which a sleeve of copper mesh is placed. The metal sleeve is again shielded by an outer shield of thick PVC material as shown in fig.

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PVC – Polyvinyl chloride

A polymer of vinyl chloride is used instead of rubber in electric cables.

The construction of the co-axial cable gives a good combination of high bandwidth and excellent noise immunity.

For 1 km cables, a data rate of 10 mbps is feasible. Higher data rates are possible for shorter cables.

Co-axial cables are widely used for cable TV and for long distance transmission within the telephone system.

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Shielding : protection of circuit from outside noise or unwanted signals.

Attenuation : Reduction of signal strength during transmission.

   Disadvantages

1. Coaxial cables are more expensive.

2.  Not compatible to twisted pair cables.

3.  Heavy as compared to twisted pair cables.

Applications of Co-axial Cables

1.             Telephone networks

2.             Cable TV

3.             Traditional Ethernet LANs

4.             Digital transmission

Fiber-Optic cable

A fiber-optic cable is made of glass or plastic and transmits signals in the form of light signals. The construction of an optical fiber cable is as shown in Fig. It consists of an inner glass core surrounded by a glass cladding, that reflects the light back into core. Each fiber is surrounded by a plastic jacket.

In fibre optics, semiconductor lasers transmit information in the form of light along hair-thin glass (optical) fibers at the speed of light, (1,86,000 miles second) with no significant loss of intensity over very long distances. As shown in fig. the system basically consists of fiber optic cables that are made of tiny threads of glass or plastic.